Abstract: The long-term goal of this competing renewal is to understand how the previously underappreciated regulation of protein synthesis (mRNA translation) in space and time drives neuronal diversity. Neuronal diversity relies on intricate steps of functional gene expression. It has been established that spatio-temporal expression of transcription factors drive neuronal and dendritic differences. While unidentified molecular mechanisms of post- transcriptional control like mRNA translation have strong potential to drive neuronal diversity, they have been thus far understudied. mRNA translation is the final essential step in the functional gene expression. We showed in the previous funding period that regulation of this process is of the key molecular mechanisms in neocortical neuronal development. Our published and preliminary studies, supported by this grant, have led to five important discoveries. First, we described that genes associated with mRNA translation show temporal dynamics in both expression and activity during neurogenesis in developing neocortices. Second, we reported that mRNA translation and the core components of the ribosome in the neocortex ? the ?neocortical ribosome signature?? are developmentally regulated by the intrinsic Elav RNA binding proteins (RBPs). Third, we published that timed ingrowth of thalamocortical axons secrete WNT morphogen and extrinsically define temporally dynamic mRNA translation and the ribosome signature in the developing neocortex. Fourth, that both Elav RBPs and thalamocortical WNT signaling dictate identities of developing neocortical glutamatergic neurons and maturation of oligodendrocytes. Finally, we reported that prenatal deletion of an Elav RBP results in abnormal neocortical dendritogenesis and behavior. However, there are still critical gaps in our knowledge regarding how timed mRNA translation and ribosome signature dictate development of distinct neocortical glutamatergic neurons. In this proposal, we hypothesize that layer-specific ribosome signatures and RBPs dictate timed mRNA translation in distinct subpopulations of developing glutamatergic neurons, thus governing their neurite development. Therefore, we will determine (1) how RBP-defined ribosome signatures dictate mRNA translation specificity and dendrite and axon development within distinct layer specific subpopulations of neocortical glutamatergic neurons; and (2) how WNT-mediated Frizzled signaling dictates RBP-defined assembly of the neocortical layer- specific ribosome signature, mRNA translation and dendrite and axon development in distinct glutamatergic neurons. To do this, we will use an elegant combination of neuroanatomical, cellular, molecular, and genetic approaches. We have produced all of the preliminary data necessary to demonstrate feasibility of the proposed approaches. Findings from this proposal will reveal previously unrecognized molecular mechanisms of post- transcriptional control in the overall specification of neocortical glutamatergic neurons and dendrite development, which can open new avenues for treatment of neurological and neuropsychiatric disorders involving these.